Optically functional materials, of which optical characteristics such as transmittance and refractive index change by exterior field such as light, temperature and electric field, have been used as elements such as optical switch, optical modulator, light amplifier, wavelength selective element, optical soliton element, optical information recording medium, temperature sensor and electro-optical element.
Examples of materials with a transmittance changeable by exterior field include such as photochromism, thermochromism and electrochromism materials. In these materials, transmittance can be changed reversibly by an application of light, temperature and electrical field, respectively, correspondingly to their intensity, and thus they are used as optical switch, temperature sensor, masking layer for optical information recording medium and the like by shielding transmitted light or controlling spot diameter of transmitted light. As examples of these materials, the following are known: Organic photochromism materials such as derivatives of triphenylmethan, azobenzene and spiropyran disclosed in JP-A-7-20600, organic or inorganic thermochromism materials such as N-salicylideneaniline, AgHgI4 and Cu2HgI4, along with materials in which these materials are dispersed in dielectric materials having different refractive indices.
On the other hand, as examples of materials with refractive index changeable by exterior field, second or third order non-linear optical materials are known, of which refractive indices change at a part applied with laser or electric field. As second order non-linear optical materials, the following are known: Organic materials such as nitropyridine derivatives and methylnitroaniline (MNA) and inorganic dielectric materials such as KNbO3 and LiNbO3, which have been used as optical switch, electro-optical element and the like. Third order non-linear optical materials include glass dispersed with semiconductor fine particles such as CuCl and CdS, which have been used as optical switch of light waveguide type wavelength selection element and light pulse generating element (See “Handbook of Optical Materials”: published by Realize Co., Ltd., 1992).
In addition, as materials with refractive index changeable with temperature, zinc based semiconductor materials such as ZnS and ZnSe and finely powdered PbS are known. Providing that amount of change in refractive index with temperature, ΔnT, is expressed by the following formula:       Δ    ⁢                   ⁢          n      T        =                        Δ        ⁢                                   ⁢        n                    Δ        ⁢                                   ⁢        T                wherein,                Δn is amount of change in refractive index; and        ΔT is amount of change in temperature these materials have been reported to have ΔnT from about 5.95×10−5/° C. to about 1.47×10−4/° C. (See Physics Report, Vol. 46, No. 12, December 1997).        
Further, as non-linear optical materials, the following are known: Those using CO3O4 and CoO as targets disclosed in JP-A-10-340482, amorphous metal oxides containing fine particles of Fe, Ni and Co disclosed in JP-A-5-224262 and substrates on which thin films of oxides of one or more elements of V, Cr, Mn, Fe, Ni, Co and Cu are formed as disclosed in JP-A-7-248516.
Since temperature sensor or device performing optical switching with temperature mainly works based on thermochromism switching as described above, amount of transmitted light varies. Therefore, they have not been suitable for a communication device in which such change gives unfavorable effects. Presently known devices made of materials whose refractive index changes with temperature have small change in refractive index and have been difficult to get sufficient change as switching or temperature sensor. Furthermore, these materials have response rate of refractive index with temperature in ms level, which is still slow in communication or light recording fields and thus have not been suitable to these applications.
Examples in the references of the above described “Physics Report” also required to be used as a dispersion in solvent due to powder form, which has made it difficult to be used as a thin film.
Furthermore, in optical switching also, these elements have had difficulty in characteristics control so as to have energy sufficient to raise change in refractive index or transmittance in these elements and to give incident laser light for stable output. The above patents do not disclose change in refractive index responsive to temperature, crystal particle size and crystal structure of oxide thin films.
Objectives of the present invention are to provide an optically functional element having large change in refractive index responsive to temperature change, its production methods and various applications using the same.